Abstract: Planktonic ciliates, which play important roles in transferring materials in food webs by grazing on primary producers, are considered as an important group of protozoans in coastal waters.Diatoms are important primary producers dominating phytoplankton community in coastal ecosystem.To better understand the role of ciliates in the transmission of primary producers (especially diatoms) along the food chain, two dominant species of tintinnids (Tintinnopsis tocantinensis and T.schotti) were collected from coastal water of Sanya bay in April, 2015, and in situ feeding diversity of these ciliates were analyzed by molecular techniques.In total, 13 diatom species belonging to 8 genera were detected in the diet of ciliates, including Skeletonema, Leptocylindrus, Rhizosolenia, Thalassosira, Pseudo-nitzschia, Cymatosira, Asterionellopsis and Chaetoceros. Two species of ciliates had the same preys as Skeletonema, Leptocylindrus, Rhizosolenia and Thalassosira, among which Skeletonema (S.costatum) was the most abundant prey items (>60%, percent of total squences).T.schotti, a larger size species, had a more diverse diet (11 species) than that in the smaller T.tocantinensis (7 species).Furthermore, the most abundant phytoplankton species (Pseudo-nitzschia) in the ambient water (49.12%, percent of phytoplankton abundance) was only detected in T.schotti(15.55%, percent of total sequences), which suggested ciliates have a selective feeding behavior in the natural environment.The results directly proved that ciliates could widely feed on diatoms and indicated that ciliates were an important link of diatom transfer along food chain in coastal ecosystem.These findings are helpful to understand the ecological role of ciliates and materials transfer in coastal ecosystems.